The molecular structure of future fuels

Future fuels will be developed from a variety of biomass and fossil sources, and must seek to address the adverse environmental impacts of current fossil fuel usage. To this end, understanding how the molecular structure of a fuel impacts on the processes of combustion and emissions production is critical in selecting suitable feed-stocks and conversion methods. This work presents experimental studies carried out on a compression ignition engine equipped with a novel low volume fuel system. This system was designed and manufactured so as that several series of single-molecule fuels, and also binary fuel mixtures, could be tested to investigate the effect of fuel molecular structure on combustion and emissions. Features of fuel molecular structure that were studied include: alkyl chain length and degree of saturation, double bond position and isomerisation and the fatty acid ester alcohol moiety. The interactions between cyclic molecules and $n$-alkanes were also studied, as was the potential of carbonate esters and terpenes as future sustainable fuels; the latter produced from genetically modified micro-organisms. The engine tests were carried out at constant injection timing and they were repeated at constant ignition timing and at constant ignition delay, the latter being achieved through the addition to the various fuels of small quantities of ignition improver (2-ethylhexyl nitrate). In tests conducted at constant injection and constant ignition timing the ignition delay of the molecule was found to be the primary driver of combustion phasing, the balance between premixed and diffusion-controlled combustion and, thereby, exhaust emissions. The various features of molecular structure were found to influence the duration of ignition delay, and an effect of interactions of binary fuel mixtures was also visible. Physical properties, such as viscosity, impacted on the production of exhaust emissions, and in extreme cases also influenced combustion phasing and heat release